The hypothesis of this proposal is that substances presented to the erythrocyte by other cells of the circulatory system cause protein phosphorylation leading to an immediate increase in O2 release and an increase in deformability of the cell. The postulate that red cell oxygen affinity is regulatable through protein phosphorylation is novel. The postulate that deformability of the cell is readily alterable is not novel but is controversial. We predict that these events occur on a time-scale that will permit increased oxygen delivery and increased blood flow through a capillary bed at a time of special need--for example, during hypoxia. At a second level of detail our hypothesis states that increased O2 release is the result of increased concentrations of 2,3 diphosphoglycerate (2,3 DPG) caused by a phosphorylation-induced decrease of pyruvate kinase activity. Changes in red cell deformability are postulated to be dependent on reversible changes in the association of certain proteins with the membrane skeleton. Membrane skeletal protein association in turn is controlled in part by the state of phosphorylation of those proteins. Our approach will be to attack the problem at 3 levels simultaneously in the expectation that progress at one level will provide insights that will increase the efficiency of work on the other levels. The first level is to observe biochemial and physical changes in the red cell in response to externally added signal molecules. The second is to study the pathways within the cell which mediate the signal-response coupling. The third level of study is to mimic the passage of erythrocytes through a capillary bed in close contact with endothelial cells and to determine effects that secretions from endothelial cells have upon the red cell under simulated in vivo conditions.